In the context of developing new long term strategies for preventing psychotic relapse among schizophrenics, increasing importance is being placed on "early" or "prodromal" signs of relapse. Since many of these signs and symptoms are non-specific and non-psychotic (i.e. anxiety, dysphoria, insomnia) it is necessary to establish their relationship to relapse. Otherwise patients may be unnecessarily or prematurely restarted on neuroleptics. In addition, the availability of techniques to predict which patients are good candidates for extended periods of drug withdrawal would be of enormous value. This study will investigate use of methylphenidate challenge test as a predictor of vulnerability to relapse following drug discontinuation and the value of prodromal signs in predicting imminent relapse. In addition to the patients who have already entered the study, seventy-three clinically stable outpatient schizophrenics who are candidates for neuroleptic discontinuation will undergo methylphenidate challenge tests and be carefully followed drug free for one year. During follow up, patients experiencing prodromal signs will be randomly assigned to drug or placebo and be intensively monitored for signs of relapse. This study will provide important information on predicting relapse in schizophrenia which would ultimately lead to maximally safe and effective long term treatment strategies.